1. Field of the Invention
The invention relates to a therapeutic method, and in particular to a method of treating cancer.
2. Description of the Related Art
Malignant tumor or cancer continuously remains the leading cause of death in humans. Specifically, for people aged less than 85 years worldwide, its mortality rate has surpassed that of heart disease. Tumor diseases comprise of myeloma, leukemia, tumor of oral cavity, pharynx, digestive system, respiratory system, bones, joints, soft tissue, skin, breast, genital system, urinary system, eyes, orbits, nervous system or endocrine system. This statistic exhibits an increase in the incidence of cancer. Thus, development of a more effective therapy for cancer is urgent.
Various procedures such as surgical, radiological, thermotherapeutic and chemotherapeutic treatments have been developed and apply to cancer therapy. Specifically, cancer treatment employing photon-activation process provides a new option for cancer therapy. This approach could be roughly categorized into two groups: photodynamic therapy (PDT) and photo-catalytic therapy (PCT). PDT uses laser, or other light sources to activate a light-sensitive agent to destroy cancer cells. The photo-sensitive agent is normally a molecular agent or drug that makes cells more sensitive to light. Once in the body, the agent is attracted to cancer cells. When the light is directed at the area of the cancer, the agent is activated and the cancer cells are destroyed. The relevant materials, apparatus and methods utilized in PDT approach have been reported by various investigators (Y. Kawai, K. Endo and M. Yoshimura, “Apparatus for treatment of cancer with photodiode,” U.S. Pat. No. 4,822,335; Fisher, G. Walter, Wachter, A. Eric and H. Graig, “Method for improved selectivity in photo-activation of molecular agents,” U.S. Pat. No. 5,829,448).
PCT comprises another branch of optical methods to treat cancer. Photocatalysis, as the name suggests, refers to catalysis under light irradiation. The most important process is photo-induced charge separation and subsequent dark catalyses by the positive and negative charges. A. Fujishima and K. Honda firstly discovered in 1972 that UV light can induce water cleavage in the presence of TiO2, which was labeled as the first photocatalytic metal oxide material (A. Fujishima and K. Honda, Nature 238:37-38 (1972)). Besides TiO2, more photocatalytic materials including both metal (SnO2, ZnO, ZrO2, CdO, In2O3, WO3 and SrTiO3 etc.) and metal chalcogenides (such as CdS, CdSe, MoS2 and WS2 etc.) have been intensively investigated. To further improve the photocatalytic efficiency, noble metals are doped to form composite photocatalytic materials (S. Vaidyanathan, E. W. Eduardo and V. K. Prashant, “Influence of Metal/Metal Ion Concentration on the Photocatalytic Activity of TiO2—Au Composite Nanoparticles,” Langmuir 19: 469-474 (2003); S. Vaidyanathan, E. W. Eduardo and V. K. Prashant, “Catalysis with TiO2/Gold Nanocomposites. Effect of Metal Particle Size on the Fermi Level Equilibration,” J. Am. Chem. Soc. 126: 4943-4950 (2004)).
Cancer treatments utilizing photocatalytic processes have been realized and practiced since the mid of 1980s. UV-illuminated TiO2 colloids were first reported to possess strong oxidation power and kill tumor cells (A. Fujishima, J. ohtsuki, T. Yamashita, S. Hayakawa, Photomed. Photobiol. 8:45-46 (1986)). Subsequent experiments contributed to the optimization of catalytic conditions by investigating the influence of superoxide dismutase (R. Cai, K. Hashimoto, Y. Kubota, A. Fujishima, Chem. Lett. 427-430 (1992)) and the interactions, between TiO2 and cells (R. Cai, H. Sakai, K. Hashimoto, Y. Kubota, A. Fujishima, Denki Kagaku 60:314-321 (1992); Y. Kubota, T. Shuin, C. Kawasaki, M. Hosaka, H. Kitamura, R. Cai, H. Sakai, K. Hashimoto, A. Fujishima, British Journal of Cancer 70:1107-1111 (1994)). Furthermore, some vivo experiments have also been conducted and confirmed remarkable cancer treatment efficacy (R. Cai, Y. Kubota, T. Shuin, H. Sakai, K. Hashimoto, A. Fujishima, Cancer Res. 52: 2345-2348 (1992); Y. Kubota, M. Hosaka, K. Hashimoto, A. Fujishima, Regional Cancer Treatment 8:192-197 (1995); A. Fujishima, T. N. Rao, D. A. Tryk, Journal of Photochem. Photobiol. C: 1: 1-21 (2000)).
Even with promising efficacy of this approach, three critical drawbacks limit its clinic applications: (1) the need for UV illumination which is not a biocompatible light source; (2) the limited penetration of UV light require optical fiber and additional surgical operation; (3) the photocatalytic treatment with titania nanoparticles is unsuitable for larger and irregular tumors.
Recently it has been demonstrated that ionization radiation such as gamma-rays could also trigger the photocatalytic process using TiO2 nanoparticles as the model photocatalyst (N. Chitose, S. Ueta, S. Seino, T. A. Yamamoto, Chemosphere 50: 1007-1013(2003)). The radiation-induced photocatalytic effects of TiO2 particles might find useful applications in environmental engineering. However, the usage of X-ray coupled with TiO2 nanoparticles in terms of photocatalytic effects have not been investigated so far. The usage of X-rays which have the most widespread applications in medicine might provide a solution to the above mentioned deadlocks encountered in UV-activated photocatalytic effect.